The Kit Selector is where you can add or build kits. Kits are a collection of up to four drums programmed within Sensory Percussion. You can think of a kit like a song or track.
You can also add existing kits to a set by dragging them from the preset kit or User Sets section of the Library panel. So if you have a particular kit from another set that you want to use (or one of the included Preset Kits), you can quickly add that kit to your current working set. Kits can be rearranged in the Kit Selector by dragging them around. You can also hold cmd while dragging to copy an existing kit, or right click on kits and empty containers to copy/paste them.
You can also paste just the MIDI data from one kit into another. This is useful if you are using MIDI to control something outside of Sensory Percussion (like lights, an external synth, or a soft synth inside a DAW) and you want to change the sounds of Sensory Percussion, but you want the same MIDI rules to apply. Just right click on the kit with the MIDI data you'd like to copy, select copy, and then target another kit in your set and select paste only midi. You can paste only midi over blank kits, or over kits that already contain sounds.
To erase a kit, right click on it in the set manager and select "clear". Be sure to save your set as a new unique title before erasing a kit if you want to access that Kit in the user sets library again later.
You can also access and drag preset drums onto the four drum channels on the mixer by double clicking on a kit in the library panel and dragging one of its drums onto the title of one of your working drum channels.